Air Marshal Edward Stringer told Sky News in his first interview since leaving the military in August that the hack had costs for the academy's operational output, forcing it to bring forward costly plans for an IT overhaul.
"The consequences for the operations were significant, but then manageable," he said, adding they were only manageable because people worked hard to keep things going.
The primary concern was that the hackers would use the academy as a "backdoor" to more secret parts of the ministry's IT systems, Stringer said. He ruled out that more breaches had happened.
An investigation was launched into the cyberattack by the ministry's digital arm, Sky News said, but no findings were made public.
Stringer said the pool of suspects was wide — from state actors, such as China, Russia, North Korea or Iran, to genuine cybercriminals looking for a vulnerability to launch a ransomware attack.